Why NOT Mistshipping though? It's certainly better than Steamshipping, and you know what, it's better than Mudshipping too. Here's why:
1) Age difference. 15 and 17 would be kinda weird.
2) Group dynamic. If you pair Mia with Isaac, then Garet gets pushed back to having less importance than he should, seeing as he and Isaac are the two guys from Vale. If Garet and Mia get paired, then Ivan is just a floating cloud, no real point in the group dynamic, because you have the hero, the couple, and then... Ivan.
If I'm wrong, please by all means, show me why.